Power transmission systems such as engines are typically used to transmit drive torque from a drive shaft to a driven shaft. A variety of such engines typically include a valve timing adjustment device. A conventional valve timing adjustment device adjusts the valve timing of the engine by opening or closing at least one of an intake valve and an exhaust valve to change the rotational phase of the driven shaft relative to the drive shaft (hereinafter simply referred to as the “rotational phase”). For example, JP-A No. 2001-41013 discloses a valve timing adjustment device including a phase change mechanism, a lever member, and a movable operating member. The phase change mechanism includes a sprocket rotating in synchronization with a drive shaft. The lever member rotates in synchronization with a driven shaft. The phase change mechanism moves the movable operating member according to a relative rotational movement between the lever member and the sprocket to change the rotational phase.
JP-A No. 2001-41013 further discloses a guide plate for controlling movement of the movable operating member. More specifically, the movable operating member fits into a guide wall of the guide plate. The moving operating member is guided in a length direction of a passage in the guide wall according to the relative rotation between the guide plate and the sprocket. The passage by which the movable operating member is guided is formed in a spiral shape. The passage includes a radial dimension extending from a rotational centerline of the guide plate that gradually increases toward one end. Therefore, when the movable operating member moves to one end of the spiral passage and away from the rotational centerline, the phase change mechanism sets the rotational phase to a delay side. Alternatively, when the movable operating member moves to the other end of the spiral passage and closer to the rotational centerline, the phase change mechanism sets the rotational phase to an advance side.
Recently, valve timing adjustment devices have been required to perform multiple operations in a short period of time. Specifically, current valve timing adjustment devices must adjust the rotational phase between the delay side immediately after startup of the engine to achieve the most delayed phase. Immediately thereafter, the valve timing adjustment device must change the rotational phase to the advance side to achieve the most advanced phase. However, the device disclosed in JP-A No. 2001-41013 cannot perform such immediate operations. A change in the direction of the rotational phase is limited by the position of the movable operating member in the passage. For example, if the movable operating member is positioned in the delay side of the passage, only a delay in the rotational phase may be promptly changed. Alternatively, if the movable operating member is positioned in the advance side of the passage, only the advance in the rotational phase me be promptly changed. To change the rotational phase from a delay to an advance or vice versa, the relative rotational direction between the guide plate and the sprocket must change. Such a change requires time that prevents the valve timing adjustment device disclosed in JP-A No. 2001-41013 from promptly changing the direction of the change in rotational phase.
Therefore, an object of the present invention is to provide a valve timing adjustment device capable of promptly switching the direction of change of the rotational phase.